Electrospray deposition is a well known method for transferring solutions into the gas phase. For example, electrospray is a practical method of ionizing samples for injection into a mass spectrometer. Electrospray-mass spectrometry has been applied to the mass analysis of high-molecular-weight compounds such as proteins, nucleotides and synthetic polymers in solution.
Electrospray can be accomplished by applying an electric field to the tip of a capillary containing an electrolyte solution at concentrations greater than about 10−6 mol/L. Under the influence of the electric field, a dipolar layer is formed at the meniscus of the solution at the capillary tip due to the partial spatial separation of the electrolyte ions. Such a dipolar layer destabilizes the meniscus and when the electric field is sufficiently high, it disperses the emerging solution into a very fine spray of charged droplets all at the same polarity. As the solvent evaporates away, the droplet size shrinks to further concentrate the droplet's charge while reducing its volume. Eventually, at the Rayleigh limit, coulombic repulsion overcomes the droplet's surface tension and the droplet explodes to form even smaller charged droplets. The process of solvent evaporation followed by coulombic explosion repeats until a mist of ion-molecules are formed. Such charged molecules may be deposited in various patterns using a mask.
Electrospray deposition of protein microarrays using a mask has been reported by Morozov and Morozova and Aveenko et al. Morozov and Morozova, Anal. Chem. 1999, 71, 1415-1420; Morozov and Morozova, Anal. Chem. 1999, 71, 3110-3117; Avseenko et al., Anal. Chem. 2001, 73, 6047-6052; Avseenko et al., Anal. Chem. 2002, 74, 927-933. In this paper, protein antigens were deposited as arrays of dry spots on an aluminized plastic surface in atmospheric condition using an electrospray technique. Dry protein spots were formed by shadow masking the ion beam through a mica sheet with array of holes (1.5 mm pitch, 30-60 um wide). A Japanese group has further defined a protein array using a photodefined fused silica as a shadow mask and a copper collimating electrode in conjunction with an electrospray deposition method. Lee et al., J. of Chem. Eng. of Japan, Vol. 36, No. 11, pp. 1370-1375, 2003.